lib-fs: If fs_get_metadata() isn't implemented, return internal metad…

…ata anyway

The metadata is sometimes used for transferring internal metadata within the
files. This metadata isn't stored to disk. So even if the fs driver doesn't
support metadata at all, it should still be possible to get/set the internal
metadata. Setting it was already possible, but getting wasn't.

src/lib-fs/fs-api.c

@@ -471,6 +471,11 @@ int fs_get_metadata(struct fs_file *file,
 	int ret;
 
 	if (file->fs->v.get_metadata == NULL) {
+		if (array_is_created(&file->metadata)) {
+			/* Return internal metadata. */
+			*metadata_r = &file->metadata;
+			return 0;
+		}
 		fs_set_error(file->fs, "Metadata not supported by backend");
 		return -1;
 	}